1. Field of the Invention
The present invention relates to systems for identifying an aircraft by ascertaining an image of its physical features, and it particularly relates to a system for orientation and scaling of a radar image whose characteristics simulate the external features of the aircraft structure.
2. Discussion of the Prior Art
Systems have been known in the past which provide images of aircraft for many reasons, including detection and early recognition of approaching enemy aircraft. Such images are useful in forming decisions to prepare for attack or, in the alternative, to decide against battle stations because interpretation of the image does not indicate that the aircraft has hostile intentions. One such system for producing radar signatures of incoming aircraft is generally called Inverse Synthetic Aperture Radar (ISAR). Unfortunately, it has been found with ISAR that the scaling and orientation of these images as seen from the reflections from the target depend largely upon the attitude of the target and its inflight changes about the roll, pitch and yaw axes.
Experimentation to provide images adequate for making command decisions in the face of an imminent enemy attack have been less than satisfactory in both processing the return information and from it deducing the physical shape of the incoming target. A basic weakness in these prior art systems has been the failure to recognize that in the orientation and scaling of the ISAR image these images are constantly changing as the attitude of the target changes. The rotation of the aircraft, for example, about the roll or longitudinal axis and relative to the radar antenna of the potential victim, causes radar observers extreme difficulty in accurately distinguishing the characteristics of the incoming aircraft. In fact, the invention recognizes that without some method of estimating the change in rate of attitude about the flight axes, most imaging radars would produce images frequently unrecognizable even by the best trained operators.